(1) Field of the Invention
This invention relates to a specular microscope for observing and/or photographing the endothelial cells of the cornea.
(2) Description of the Prior Art
There are hexagonal cells having a size of about 20 .mu.m called endothelial cells behind the cornea of the human eye, these play a vital role in maintaining the transparency of the cornea. As surgical microscopes, etc., have been developed and become wide-spread in recent years, techniques of eye surgery have also made remarkable progress, and surgical operations such as of the vitreous body, ultrasonic surgical operations of cataracts, implantation of corneas, implantation of artificial crystalline lenses, and the like are carried out daily. Since surgical instruments come into contact with the eye being operated on, and perfusion solutions such as physiological salt solutions, Ringer's solution, or the like affect the endothelial cellular membrane of the cornea, the transparency of the cornea is reduced after such a surgical operation and this causes the problem of a drop in the visual acuity. For this reason, it has become important to observe and/or photograph the endothelial cells of the cornea in vivo so as to measure and inspect their changes.
To comply with this demand, an instrument called a specular microscope has been developed and put into practical application as an apparatus for observing and photographing the endothelial cells of the cornea. In the specular microscope, illuminating light beams are projected onto the cornea through a slit-illuminating optical system so as to form a slit image on the endothelial cell layer of the cornea. The illuminating light beams are subjected to mirror reflections at the endothelial cell layer of the cornea, and are emitted from the eye to form reflected light beams. The specular microscope is designed to detect these reflected beams to observe and photograph an image of the endothelial cells of the cornea.
The cornea is usually a light-transmitting member and the reflectivity of the endothelial cells is extremely low. For this reason, the specular microscope is designed so that it utilizes the mirror reflection at the endothelial cell layer of the cornea for observing and photographing the endothelial cells, as described above. To this end, the slit-illuminating optical axis and the optical axis of the reflected light beams are symmetrical with each other with respect to a normal line the illuminated member, that is, to the portion being observed.
However, it should be noted that the reflectivity of the surface of the cornea is by far greater than that of the endothelial cells of the cornea. In fact, the reflectivity of the endothelial cell layer is approximately 0.02% whereas the reflectivity of the surface of the cornea is approximately 2.5%. Moreover, the cornea is extremely thin. For these reasons, the light beams reflected at the surface of the cornea enter the visual field of observation of the specular microscope. As a result, the image of the light reflected at the upper surface of the cornea partially overlaps the image of the endothelial cells and the image of the light scattered from the cornea. Thus, there is produced a critical problem that observation of the endothelial cells of the cornea is adversely affected.
It should further be noted that conventional specular microscopes have been designed as a mono-functional single-purpose instrument which can only be used for observation and photography of the endothelial cells of the cornea, regardless of whether it is of a contact type or a non-contact type. In view of the fact, a proposal has been made by Shiro Takizawa and Shinichi Nishimura in U.S. patent application Ser. No. 122,673 dated Feb. 19, 1980 now U.S. Pat. No. 4,396,260 which is assigned to the same assignee as the present application on a slit lamp which can also be used as a specular microscope simply by replacing the objective lens of an observation microscope of the slit lamp by an attachment for a specular microscope. This slit lamp is characterized by the fact that the illumination of the endothelial cells of the cornea is effected through the slit-illuminating system of the slit lamp. The proposed instrument has been found disadvantageous in that it is difficult to position the optical axis of the observation microscope and the optical axis of the slit-illuminating system symmetrical with each other with respect to the normal line of the portion being inspected. Thus, it is very difficult to establish an alignment in which mirror reflection at the endothelial cells is introduced into the observation microscope.
Furthermore, in view of the fact that whether or not the endothelial cells of the cornea have been modified by a surgical operation is determined by calculating the number of endothelial cells per unit area or by measuring the size of individual cells or the distribution of cell sizes, graduated lines are provided within the observation microscope of the specular microscope, or graduated lines are simultaneously photographed together with the endothelial cells. The conventional graduated lines are formed by vacuum evaporation of aluminum onto a glass plate and appears as black lines within the visual field so that it is difficult to distinctly observe them the faint images of the endothelial cells. Since the graduated lines must be photographed so that their images are produced over the images of the endothelial cells when a photograph is to be taken, as that the images of the graduated lines become an obstacle to the resulting photograph.